The self fit kind are a replacement for a sports mouth guard (hockey, rugby, football) - protect your upper teeth only. They also help with teeth grinding (bruxism). A dentist will sell a custom fit version for several hundred dollars. (Both are useful for their need, the custom fit are more comfortable, but at a 10-20x price hardly worth it).

This article is about a guard that connects to your upper and lower teeth, and draws your jaw forward (it has to to help with obstructive apnea). You basically can't talk while wearing, it takes getting used to. They're not always comfortable, but they definitely help. Less invasive than a CPAP, and you don't need a machine/constant power source.

But there's multiple types of apnea, which the paper[0] makes clear (it's only talking about obstructive apnea) - this headline suggests guards are better for all. That's far from true. Obstructive is more common, but; for the other kind (where you forget to breath at night) this does nothing, and a CPAP is the only solution.

[0]: https://www.sciencedirect.com/science/article/pii/S073510972...